1. Field of the Invention
The present invention relates to a method of preparing a refractive surface of a progressively variable focal power ophthalmic lens, comprising the steps of defining a curve having a curvature which varies progressively according to a predetermined law along at least a part of said curve, defining a first family or surfaces admitting said curve as a main meridian curve, so that each surface of the first family comprises an upper zone having a first substantially constant focal power for distant vision, a lower zone having a second substantially constant focal power for near vision and an intermediate zone situated between the upper and lower zones and connected continuously therewith, said intermediate zone having a focal power which varies progressively from the first focal power at its upper limit to the second focal power at its lower limit according to said predetermined law along said part of the main meridian curve, which extends substantially vertically across the three zones in their middle and is an umbilic curve at least in the intermediate zone, the difference between said second power and said first power being called the power addition, in drawing up for each surface of the first family a table of deviations in relation to a reference sphere and a chart of the astigmatism aberrations, in choosing among the surfaces of the first family those whose astigmatism aberrations are concentrated in the outer lateral parts of the intermediate zone.
2. Prior art
Progressively variable focal power ophthalmic lenses are already known, of which one of the two refractive surfaces has a structure similar to that described above.
The upper and lower zones have a spherical shape, and the curves of intersection of the intermediate zone of the refraction surface by planes perpendicular to the main meridian curve have a circular shape. Such surfaces are for example described in French Pat. Nos. 1,095,375 and 1,544,799. The refractive surfaces described in these two French patents have advantage of having an intermediate zone whose median part, i.e. the part situated on each side of the main meridian curve and in the vicinity thereof, does not present or practically not any astigmatism aberration. However, in the lateral parts of the intermediate zone, the astigmatism aberrations and the oblique distortion increase rapidly further away from the main meridian curve. This may be illustrated by observing a grid-shaped object through the lens. It may in fact be seen that vertical and horizontal lines of the object which are seen through the lateral parts of the intermediate zone of the refraction surface are very greatly distorted.
This disadvantage is overcome when an ophthalmic lens is used having a surface of refraction such as that shown in FIGS. 7 and 8 in French Pat. No. 1,095,375. In this case, the progressively variable focal power intermediate zone and the substantially constant focal power lower zone for near vision are formed only in the median part of the refractive surface and the upper substantially constant focal power zone for distant vision is extended downwards to the lower edge of the refractive surface on each side of said median part. Since the upper zone and its downward lateral extensions have a constant power, i.e. have a spherical shape, we obtain a lens whose lateral parts present no astigmatism aberrations nor oblique distortion. That is to say that the vertical and horizontal lines of a grid-shaped object seen through the lateral parts of the lens remain vertical and horizontal. However, as can be clearly seen in FIG. 8 of French Pat. No. 1,095,375, the refractive surface of such a lens presents inevitably at the boundary between the progressively variable focal power median part and each of the two downward lateral extensions of the upper zone a discontinuity or facet in the shape of a step parallel to the plane of the main meridian curve, which is unaesthetic. Furthermore, the horizontal lines of the image of the grid seen through the lens present considerable shift in the vertical direction at the position of the facets, which is extremely awkward for the wearer of spectacles when he goes from looking through said median part to looking through one of said lateral parts or vice-versa.
So as to attenuate the oblique distortion of a progressively variable focal power ophthalmic lens, it has also been proposed to construct the refractive surface so that the horizontal sections of said refractive surface, i.e. its sections through planes perpendicular to the main meridian curve, comprise a single section of circular shape, the other horizontal sections having the shape, or substantially the shape, of conic section portions, i.e. the shape of portions of ellipses, hyperbola or parabola, whose radius of curvature increases in the direction away from the main meridian curve when the radius of curvature at the point of intersection of the conic section considered with the main meridian curve has a smaller value than the value of the radius of the circular shaped section, and whose radius of curvature decreases in the direction away from the main meridian curve when the radius of curvature at the point of intersection of the conic section considered with the main meridian curve has a value greater than the value of the radius of the circular shaped section. Furthermore, the refractive surface may comprise in its upper part and in its lower part at least one horizontal line which is an umbilic curve or a curve along which the vertical component of the prismatic effect has a constant value, i.e. a curve at each point of which the plane tangential to the refractive surface forms a constant angle with the horizontal plane which contains the optical center of the lens and which is perpendicular to the main meridian curve. The refractive surface may furthermore comprise in its lateral parts at least one vertical line along which the horizontal component of the prismatic effect is constant, i.e. at each point of which the plane tangential to the surface of refraction forms a constant angle with the plane, of the main meridian curve. See in this connection French Pat. No. 2,058,499 and its first Certificate of Addition No. 2,079,663. With such a structure of the refractive surface, it is possible to reduce to a large extent the oblique distortion, but this reduction of the oblique distortion is obtained at the price of a different distribution of the astigmatism aberrations over a greater area of the surface.